Repository authentication

To use authenticated apt repositories, multistrap either needs to
be able to install an appropriate keyring package from the existing
apt sources outside the multistrap environment or have the
relevant keys already configured using apt-key on the host system.

If relevant packages exist, specify them in the keyring
option for each repository. multistrap will then check that apt has
already installed this package so that the repository can be
authenticated before any packages are downloaded from it.

Note that all repositories to be used with multistrap must
be authenticated or apt will fail. Similarly, secure apt can only be
disabled for all repositories (by using the --no-auth command line
option or setting the general noauth option in the configuration
file), even if only one repository does not have a suitable keyring
available. Not all packages need keyring packages, if you configure
apt-key appropriately.

The keyring package(s) will also be installed inside the multistrap
environment to match the installed apt sources for the multistrap.

All configuration of apt-key needs to be done for the machine
running multistrap itself.